Genetics of Northern krill (Meganyctiphanes norvegica Sars)

Understanding the origin and maintenance of genetic diversity in the oceanic realm is difficult because barriers to gene flow are far less obvious in marine compared to terrestrial species. This is particularly so for planktonic species such as euphausiids, with no fossil record and high rates of dispersal, and in which paleobiology and evolutionary history remains largely obscure. Population genetics may play an important role in this respect, elucidating population connectivity and shedding light on the historical demography of the investigated species. In turn, the relevant factors that can promote speciation over both at short and long evolutionary timescales can be identified. In this chapter, we outline the available approaches for gathering population genetics information on marine organisms, with a particular focus on the recent achievements in the study of Meganyctiphanes norvegica. For population structure, we review the data available for mitochondrial DNA (mtDNA) markers that show the presence of four temporally stable and genetically distinct gene pools, one in the Mediterranean samples and three others in the North Atlantic Ocean, potentially associated with the basin-scale pattern of circulation. Unpublished data on some nuclear microsatellite markers adds support to this conclusion. In addition, we apply a newly introduced Bayesian coalescent approach, and demonstrate that previously reported mitochondrial sequence diversity is indicative of a recent expansion at the Northern edge of the species' distribution. This does not hold for the Southern and Mediterranean populations that appear to be stable over time. We also review the literature reporting new advances on the analysis of M. norvegica genes and genes' products involved in metabolic pathways that may underline differences at the population level, possibly linked to environmental variation and local adaptations.